1. Field of the Invention
The present invention relates to a suction roll for dewatering a material web guided on a transport belt in a paper machine. The suction roll includes a roll jacket over which the transport belt is guided, and a first external suction box that partially encompasses an outer periphery of the roll jacket and that is adapted to create a vacuum that suctions a gusset formed between the material web and the roll jacket.
2. Discussion of Background Information
A suction roll of the type generally discussed above is known, e.g., from GB-A-713 190. In the known arrangement, a felt guides a material web over a guide roll and through downstream nips formed by an upper press roll and two lower press rolls. The first nip is formed by a lower press roll composed of a suction roll. A first suction box is positioned in front of the suction roll and is sealed against the suction roll and against the felt in a region between the gusset and the guide roll. A second suction box extends into the two gussets formed between the upper press roll and the suction roll and between the upper press roll and the other, i.e., downstream, lower press roll, and is sealed at the bottom against the surface of the suction roll and the downstream press roll.
The dewatering effect of the suction roll can be improved with the first suction box, however, sealing of the suction box against the felt in the vicinity of the inlet gusset and in the vicinity of the upstream guide roll is not adequately achieved by a sealing plate that extends over an entire width of the suction box and that extends from the upstream guide roll to a region just in front of the inlet gusset. A wide sealing plate of this type is complex and expensive, and, because it abrades the felt, is not particularly well suited for high web running speeds. Moreover, the second suction box is also complex and expensive in that it is difficult to mount due to the narrowed space conditions.
A further problem associated with known suction rolls of the type generally discussed above is that unwanted air is sucked in. In an attempt to avoid this problem, upstream suction boxes have been utilized in rolls discussed in, e.g., CH-A-608 257, AT-A-340 763, DE-A-2 741 144, DE-A-311 570, or U.S. Pat. No. 3,291,680.
In these publications, the suction roll is encompassed on both sides by the suction box, see, e.g., AT-A-340 763. However, this arrangement also suffers from the same drawbacks discussed above, e.g., suctioning unwanted air.